Thermal energy storage system

ABSTRACT

A thermal energy storage system in which at least one vertically extending cavity is formed within ground constituted by geologically stable consolidated rock, and at least one cylindrical steel vessel having a diametral dimension smaller than its longitudinal dimension is positioned within the cavity and surrounded peripherally by a containment material. Conduits are provided for directing pressurised water in vapour and/or liquid phase into the vessel and for conveying steam from an upper region of the vessel. The vessel has a peripheral wall acting as a liner for the containment material and internal pressure-induced forces are transferred from the vessel to the containment material via the peripheral wall. The containment material in one embodiment of the invention includes the surrounding rock. In a further embodiment the containment material includes a filler material and the internal pressure induced forces are transferred from the vessel to the surrounding rock via the filler material.

FIELD OF THE INVENTION

This invention relates to a thermal energy storage system for use inassociation with steam raising plant. The invention might be employed,for example, in association with such plant as nuclear reactors andpackage boilers that may be required to meet transient peak demands thatexceed the steady state output capacity of the plants. A furtherapplication of the invention is in conjunction with solar energycollector systems of the type that are employed for converting solarenergy to thermal energy.

BACKGROUND OF THE INVENTION

A solar energy collector system that is employed in the generation ofthermal energy comprises a Linear Fresnel Collector (“LFR”) system. Thissystem employs a field of reflectors and elevated energy receivers thatare illuminated by reflected radiation for energy exchange with fluidthat is carried through the receivers, and the thermal storage system ofthe present invention is hereinafter described by way of example in thecontext of an LFR system.

The LFR system is typically employed in the production of steam fordelivery to electricity generating plant, either for admission directlyto steam turbines or for heat exchange with a working fluid. However,the LFR and other solar collector systems are functional only in thepresence of adequate incident solar radiation and, in order to prolongthe duty cycle of solar-based electricity generation (and so minimisethe demand on parallel sources of energy), thermal energy produced inexcess of demand during periods of high-level solar radiation and/or lowpower consumption must be stored. Three storage systems have previouslybeen proposed for this purpose; one involving the use of pre-existing orpurpose-built deep subterranean cavities, the second involving theemployment of an above-ground pressure vessel and the third involvingthe use of concrete-encased fluid feed pipes.

Subterranean cavity storage of water at required temperature andpressure, typically 270° C. to 340° C. at 100 to 200 Bar, offers certainadvantages, in that surrounding rock provides a natural “pressurevessel” and large storage volumes can be accommodated. This type ofstorage currently is employed for combustible gases, for example LPG;but for high temperature water storage it would be necessary tocompletely line the cavity with an impermeable water-rock interface andthe cavity would need be located at a depth that provides for a rocksurface stratum of thickness sufficient to withstand the high fluidpressure within the cavity. These requirements impose formidableconstruction and costing constraints on deep cavity storage.

Above ground pressure vessels that are suitable for containing water atthe required temperature and a pressure sufficient to maintain the fluidin a liquid phase have been built for various purposes and are commonlyreferred to as “steam accumulators”. However, the fabrication andmaterial costs inherent in building a vessel having the volumetriccapacity required for storage of sufficient water to provide the steammass flow rate required to sustain power generation, for example 1 to 5MW for a period of 3 to 24 hours, has been determined to bedisproportionately high relative to other components of the total powergenerating system.

In the third type of storage, cracks occur in the concrete encasementdue to differential expansion between the concrete and the pipes. Thisleads to the appearance of gaps between the pipes and the concrete and,consequently, poor heat transfer (in both directions) between the pipesand the concrete. Cracks in the concrete also cause thermal islandswhich cannot usefully participate in thermal storage.

SUMMARY OF THE INVENTION

Broadly defined the present invention provides a thermal energy storagesystem that comprises at least one vertically extending cavity formedwithin ground that is constituted by geologically stable consolidatedrock. At least one cylindrical steel vessel having a diametral dimensionsmaller than its longitudinal dimension is positioned within the atleast one cavity and surrounded peripherally by a containment material.Conduits are provided for directing pressurised water (in vapour and/orliquid phase) into the vessel, and for conveying steam from an upperregion of the vessel. The vessel has a peripheral wall that functions asa liner for the containment material whereby, in operation of thesystem, internal pressure-induced forces are transferred from the vesselto the containment material by way of the peripheral wall.

The invention is further defined as providing a method of storingthermal energy wherein water at a high temperature is maintained underpressure within a cylindrical steel vessel which is positioned within acavity which is formed within ground that is constituted by geologicallystable consolidated rock, wherein the vessel is surrounded peripherallyby a containment material and wherein the vessel has a peripheral wallthat functions as a liner for the containment material and internalpressure-induced forces are transferred from the vessel to thecontainment material by way of the peripheral wall.

Within the context of the present invention the term “cylindrical steelvessel” is to be understood as meaning a vessel having any desiredcross-sectional configuration (for example, circular or hexagonal) butone that is substantially constant along its length.

OPTIONAL FEATURES OF THE INVENTION

The containment material optionally comprises surrounding rock and, insuch case, the invention in one of its aspects may be defined asproviding a thermal energy storage system that comprises at least onevertically extending cylindrical cavity formed within ground that isconstituted by geologically stable consolidated rock, the cavity havinga diametral dimension that is substantially smaller than the cavity'slongitudinal depth. A cylindrical steel vessel is positioned within thecavity and conduits are provided for directing pressurised water (invapour and/or liquid phase) into the vessel, whereby water is maintainedwithin a major volumetric portion of the vessel, and for conveying steamfrom an upper region of the vessel. The vessel is dimensioned tofunction as a liner for the cavity and, in operation of the system, totransfer internal pressure-induced forces to the surrounding rock.

In functioning as a liner and, consequently, preventing movement ofstored water into the surrounding rock, the vessel substantiallyeliminates the possibility of explosive rock fracturing that mightotherwise occur with generation of steam pressure in pores and defectswith conductive heating of the rock to temperatures above 100° C.

In a further embodiment the containment material optionally comprises afiller material and, in this case, the invention in accordance withanother of its aspects may be defined as providing a thermal energystorage system that comprises a vertically extending cavity formedwithin ground that is constituted by geologically stable consolidatedrock. At least one cylindrical steel vessel is positioned verticallywithin the cavity, and conduits are provided for directing pressurisedwater (in vapour and/or liquid phase) into the vessel and for conveyingsteam from an upper region of the vessel. Also, a thermally stablefiller material is located between the vessel and a surrounding wall ofthe cavity. The vessel has a diametral dimension that is substantiallysmaller than the vessel's longitudinal length and the wall of the vesselfunctions as a liner for the filler material whereby, in operation ofthe system, internal pressure induced forces are transferred from thevessel to the surrounding rock by way of the filler material.

By “thermally stable filler material” is meant one that maintains itsphysical and chemical properties when exposed to operating temperaturesof the storage system, for example temperatures of the order of 200° C.to 420° C.

In forming the vessel with a diametral dimension that is substantiallysmaller than its longitudinal length, the pressure-induced forcesexerted on the ends (and, more relevantly, on the uppernear-ground-surface end) of the vessel are reduced to a level that canreasonably be accommodated. This in turn permits practicable end-cappingand securement of the upper end of the vessel against high internalpressures, typically of the order of 80 to 200 Bar. Also, in the case ofthe second aspect of the invention, by dimensioning the vessel as aliner for the filler material, the surrounding filler material and rockare (jointly) effectively integrated as side and lower end walls of thevessel. This in turn permits fabrication of the circumferential wall andlower end of the vessel from relatively small gauge (typically 6 mm to16 mm thick) steel.

The cavity may be formed in ground constituted by high strength, lowporosity rock such as granite. However, it has been determined thatcertain advantages are to be gained from location of the cavity in arock mass having the stiffness of Triassic sandstone. But, because thisrock typically has a porosity of 15% to 25%, care should be taken toform the cavity in dry rock above the level of any underlying watertable. The cavity should be formed with a depth sufficient to locate thecontained vessel below ground coverage and, thus, wholly withinsurrounding consolidated rock.

The size of the vessel will be determined by the required storagevolume.

This might be of the order of 60 m³ for approximately 5 hours of storagefor a 1 MW heating module, and for this capacity the vessel may havedimensions of the following order:

About 3.0 m diameter and about 9 m length,

about 2.0 m diameter and about 20 m length,

about 1.5 m diameter and about 36 m length, or

about 1.0 m diameter and about 75 m length.

However, when the thermal storage requirement exceeds that which mightoptimally be met with a single conveniently sized vessel, the storagemay be provided by a plurality of parallel vessels, disposed in a matrixof the filler material, either in a grid formation with regular mutualseparation or in a cluster. When disposed in a grid formation eachvessel may have a diameter of approximately 1.5 m and a depth in therange of 6 m to 36 m. A typical inter-vessel spacing of about 3 m maythen be employed, with the filler material between adjacent vesselsbeing loaded in compression by the radial forces attributable topressure within the vessels. When disposed in a cluster, the vessels maybe formed with a triangular, square, hexagonal or other polygonalcross-sectional configuration that permits close packing of the vessels.They may alternatively be formed with a circular cross-section and beclose-packed with the filler material occupying interstitial spacesbetween the vessels.

When close-packed as above described, the vessels when heated willexpand effectively as a unit, so it is necessary that the surroundingfiller material be constituted or structured in a way to accommodate thetotal effective expansion.

With these various arrangements, heat within the boundary of the totalstorage region will substantially be conserved and, allowing for thehigh rock-to-water relative heat capacities, the cavity-defining rockwill make a useful dynamic contribution to the thermal storage. Also,the filler material may be selected to provide a high fillermaterial-to-water relative heat capacity and so add a further usefuldynamic contribution to the thermal storage.

Thermal expansion of the vessel or, if more than one, each vessel may beaccommodated by selecting the filler material as one having acoefficient of thermal expansion approximately the same as that of thesteel from which the vessel is fabricated or as one that exhibitsresiliency sufficient to compress and expand with change in diameter ofthe vessel. The filler material might comprise a compressible materialsuch as cork or other vegetable material, or a mineral material ineither solid or particulate form. The filler material will be selectedto withstand prevailing storage temperatures and, when in the form of amineral, the filler material may comprise solid concrete or discreteparticles of, for example, a thermally conductive material, capped inthe latter case to prevent vertical displacement of the material. Whenin the form of concrete it may be laminated with a layer of acompressible material, either within the concrete or at theconcrete/vessel interface. In the latter case, the compressible materialmay be wrapped about a vessel prior to placing the vessel in positionand prior to surrounding the vessel with such other filler material asconcrete. When the vessels are positioned in close-packed relationship,a compressible material may be positioned between adjacent vessels

The complete thermal energy storage system may be configured to providefor receipt and liberation of thermal energy by and from the vessel(s)either directly or by way of heat exchangers. Heated fluid to and flashsteam from the (or each) vessel may be channelled through separate,parallel circuits or, in one application of the invention, by way of aseries circuit incorporating a steam turbine, a condensate reservoir anda solar energy collection system.

The arrangements comprising a plurality of vessels may be employed toprovide for storage volume adjustment and temperature control undervariable load demands. This may be achieved by interconnected valvingand pumping of at least some of the vessels.

Temperature and pressure conditions in a multi-vessel system may bemaintained substantially constant across all vessels, by connecting allvessels in parallel to a common input header, to create uniform inputtemperature conditions, and by connecting all vessels in parallel to acommon output header, for maintenance of uniform vessel pressure. Thecommon output header will allow flow of steam between vessels, leadingto temperature equalisation but possible water volume differences.However, the vessels may be interconnected to permit gravitationaladjustment of water level differences.

The heating system that is employed to generate the thermal energy to bestored in the thermal energy storage system as above described mayoptionally comprise or incorporate any known type of heating system,such as for example a fossil-fuel-fired boiler or anuclear-reactor-powered plant that is arranged to exchange heat with theworking fluid. However, in one embodiment of the invention the heatingsystem comprises a solar energy collector system in which incident solarradiation is reflected to illuminate receivers through which the workingfluid is passed.

When the heating system comprises a solar energy collector system, suchsystem may optionally incorporate at least one field of reflectorswithin which a plurality of receivers is located. Each receiver may beassociated with a single reflector, for example a parabolic troughreflector, or each receiver may be associated with and receive reflectedradiation from a plurality of reflectors. In the latter case thereflectors may comprise heliostats having either horizontal or verticalfixed axes, or linear reflectors. In each of these possible systems theworking fluid is directed through tubes within the receivers and isheated by concentrated solar radiation from the reflectors.

The invention will be more fully understood from the followingdrawing-related description of illustrative embodiments of the thermalstorage system and associated plant.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings—

FIG. 1 shows a schematic representation of a power generating plantincorporating an arrangement of a solar energy collection system, asteam turbine and a thermal energy storage system having a single groundcavity.

FIG. 2 shows a schematic representation of a power generating plantincorporating a series arrangement of a solar energy collection system,a steam turbine, a condensate reservoir and a thermal energy storagesystem having plural ground cavities,

FIG. 3 shows a diagrammatic representation of a ground cavity and acontained water storage vessel,

FIG. 4 shows a diagrammatic representation of an alternativecapping/securing arrangement for the upper end of the vessel shown inFIG. 3,

FIG. 5 shows (in plan) an example of a grid arrangement of pluralcavities,

FIG. 6 shows a diagrammatic representation of a ground cavity containinga water storage vessel and, in accordance with a second embodiment ofthe invention, a filler material,

FIG. 7 shows (in plan) an example of a grid arrangement of plural waterstorage vessels located within a filler material matrix which is, inturn, located within a ground cavity,

FIG. 8 shows a representative cluster of hexagonal-section water storagevessels located within a filler material matrix,

FIG. 9 shows a representative cluster of circular-section water storagevessels located within a filler material matrix, and

FIG. 10 shows a schematic representation of a solar energy collectionssystem portion of the power generating plant.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, the power generating plant incorporates asolar energy collector system 10, a steam turbine 11 coupled to anelectrical generator 11 a and a thermal energy storage system 12.Ancillary equipment, such as valves and metering devices, as wouldnormally be included in such a plant have been omitted from the drawingsas being unnecessary for an understanding of the invention. So too havebeen connections and valving arrangements that might be provided forbypassing the thermal storage system and directly feeding the steamturbine from the solar collection system and alternative energy sources.

As shown diagrammatically in FIG. 10 the solar energy collection system10 comprises a field of arrayed ground-mounted, pivotal reflectors 13that are driven to track the sun and, in so doing, to reflect incidentsolar radiation to illuminate an elevated receiver system 14. Thereflectors might typically comprise units as disclosed in InternationalPatent Applications PCT/AU2004/000883 and PCT/AU2004/000884, and thereceiver system might typically comprise a system as disclosed inInternational Application PCT/AU2005/000208.

Water at a temperature of approximately 180° C. and under pressure ofthe order of 100 Bar is delivered to the solar energy collection system10 from a lower region of the thermal storage system 12 by a conduit 15and, after gaining heat in the energy collection system, is returned toan upper region the thermal storage system 12 by way of a pump 16 andconduit 17. The pump 16 is driven to adjust for any pressure drop in thecircuit and to provide for any required pressure boost. In a typicalsystem and under optimum conditions of solar heating, the heated wateris returned to the thermal storage system 12 at a temperature within therange 220° C. to 360° C., with the pressure controlled by the pump 16 tomaintain the major portion of the fluid in the thermal storage system 12in its liquid phase.

Flash steam that is released (as required) from the upper region of theenergy storage system is admitted to the steam turbine 11 at atemperature typically in the range of 215° C. to 340° C. by a conduit 18and, after expanding through the turbine, is returned to the lowerregion of the storage system by way of a conduit 19 and a pump 20.

The system as illustrated in FIG. 2 is conceptually similar to that ofFIG. 1 and like reference numerals are used to identify like components.However, in this embodiment flash steam from the upper region of each oftwo thermal storage systems 12 is conveyed to the turbine 11 by aconduit 21, and after expanding through the turbine the resultant vapouris directed into a ground level series-connected condensate reservoir23. The reservoir accommodates fluctuations in the water level in thethermal storage system 12 and provides for balancing of water transportthroughout the system. Water, typically at about 30° C. to 50° C., isconveyed to the solar energy collection system 10 by way of a pump 24and conduit 25 where it is heated to a temperature in the range of 270°C. to 340° C. and returned via conduits 26 and pumps 27 to the lowerregion of the thermal storage systems 12, under a pressure of about 70to 150 Bar.

FIG. 3 provides a diagrammatic illustration of a single thermal storagesystem 12 which comprises a vertically extending cylindrical cavity 28which is formed by boring or by any other suitable excavation processwithin ground that is constituted by geologically stable consolidatedrock 29. The cavity 28 has a diametral dimension that is substantiallysmaller than the cavity's longitudinal depth, and a cylindrical steelvessel 30 that holds the pressurised water is positioned within thecavity at a level below that of unconsolidated ground cover 31. Thevessel 30 is formed with a relatively thin wall, having a thickness inthe range 6 mm to 16 mm over a major portion of its extent, and thevessel is otherwise dimensioned to be a neat fit in the cavity 28 andthus to function as a liner for the cavity.

The lower end of the vessel is formed with a convex end portion 32 thatis welded to the cylindrical wall portion 33 of the vessel, and thesupporting rock 29 is formed with a complementary concavity for nestingthe end portion. With this arrangement internal pressure-induced forcesare transferred radially and downwardly through the vessel wall to thecontaining rock.

As illustrated in FIG. 3, the upper end of the vessel is also formedwith a convex end portion 34 that is welded to the cylindrical wall.However, allowing for the fact that there is no rock containment of theupper end of the vessel, the convex end portion 34 will be formed fromsteel having a thickness typically of at least 20 mm.

As an alternative and as shown in FIG. 4, the upper end of the vesselmay be closed by a heavy clamp 35 a that is anchored to the rock stratumby rock bolts 35 b. This approach might be adopted when, for example,larger diameter cavities are employed and greater upward forces would beapplied to the welded end portion of the vessel.

The steel vessel 30 may be prefabricated, either partly or wholly,before being located in the ground cavity 28. Alternatively, the vesselmay be fabricated in situ by welding successive cylindrical sectionstogether and by lowering the vessel into the ground cavity asfabrication progresses. When the installation has been completed andfluid connections have been made, the upper end of the vessel is coveredwith removable backfill, and roofing or another form of covering (notshown) may be employed to direct rain away from and/or to shield thearea of hot ground.

As indicated previously, the thermal energy storage system may comprisea plurality of ground cavities 28, each occupied by its own storagevessel 30, and in such case the cavities may be arranged in a gridpattern such as shown in FIG. 5.

FIG. 6 provides a diagrammatic illustration of a second embodiment of athermal energy storage system 12 that is similar to that described withreference to FIG. 1 and like reference numerals are employed todesignate like parts. However, in this case a thermally stable fillermaterial 36 in the form of cork sheeting, a particulate material, amortar, a compressible concrete or other compressible material islocated about the vessel, and the vessel wall thus functions as aninternal liner for the filler material.

As indicated previously, the thermal storage system may comprise aplurality of the water storage vessels 30 located within a single groundcavity 28. In one such case the vessels may be arranged in a gridpattern and be positioned within a matrix of the filler material 36 inthe form of concrete or particulate material as shown in FIG. 7. Withthis arrangement each vessel 30 forms a liner for the filler material36, and the filler material surrounding and between adjacent vessels isloaded in compression by the radial forces attributable to pressurewithin the vessels.

In an alternative arrangement, as indicated in FIG. 8, the water storagevessels 30 may be clustered in a close-packed contacting arrangementwithin a surrounding filler material 36. In order to achieve this resulteach of the vessels may be formed with a hexagonal cross-section, asindicated, or with such other cross-section, such as a triangularcross-section, as permits their close packing.

FIG. 9 illustrates a further possible close-packed arrangement ofmultiple water storage vessels 30, in this case with circular sectionvessels positioned within a matrix of filler material 36 and with thefiller material located in the interstices between adjacent vessels.

The heating system 10, in the form of a CLFR solar energy collectorsystem 20, is illustrated in a diagrammatic way in FIG. 10. Theillustrated solar energy collector system comprises a field of arrayedground-mounted, pivotal reflectors 13 that are driven to track the sunand, in so doing, reflect incident solar radiation to illuminate anelevated receiver system 14. In the form illustrated, the reflectors 13pivot about horizontal axes.

Variations and modifications may be made in respect of the invention asabove described and defined in the following claims.

1. A thermal energy storage system comprising at least one verticallyextending cavity formed within ground that is constituted bygeologically stable consolidated rock, at least one cylindrical steelvessel having a diametral dimension smaller than its longitudinaldimension positioned within the at least one cavity and surroundedperipherally by a containment material, conduits provided for directingpressurised water (in vapour and/or liquid phase) into the vessel andfor conveying steam from an upper region of the vessel, the vesselhaving a peripheral wall that functions as a liner for the containmentmaterial whereby, in operation of the system, internal pressure-inducedforces are transferred from the vessel to the containment material byway of the peripheral wall.
 2. A thermal energy storage systemcomprising at least one vertically extending cylindrical cavity formedwithin ground that is constituted by geologically stable consolidatedrock, the cavity having a diametral dimension that is substantiallysmaller than the cavity's longitudinal depth, a cylindrical steel vesselpositioned within the cavity and conduits provided for directingpressurised water (in vapour and/or liquid phase) into the vessel andfor conveying steam from an upper region of the vessel, the vessel beingdimensioned to function as a liner for the cavity and, in operation ofthe system, to transfer internal pressure-induced forces to thesurrounding rock.
 3. A thermal energy storage system comprising avertically extending cavity formed within ground that is constituted bygeologically stable consolidated rock, at least one cylindrical steelvessel positioned vertically within the cavity, conduits provided fordirecting pressurised water (in vapour and/or liquid phase) into thevessel and for conveying steam from an upper region of the vessel, athermally stable filler material located between the vessel and asurrounding wall of the cavity, the vessel having a diametral dimensionthat is substantially smaller than the vessel's longitudinal length andthe wall of the vessel functioning as a liner for the filler materialwhereby, in operation of the system, internal pressure induced forcesare transferred from the vessel to the surrounding rock by way of thefiller material.
 4. The thermal energy storage system as claimed inclaim 3 wherein the thermally stable filler material comprises amaterial that maintains its physical and chemical properties whenexposed to operating temperatures of the storage system.
 5. The thermalenergy storage system as claimed in claim 3 wherein the filler materialis selected from a material that has the capacity to accommodate thermalexpansion of the vessel.
 6. The thermal energy storage system as claimedin any one of claims 1 to 3 wherein the steel vessel has circularcross-section and a diameter within the range of about 1.0 m to about3.0 m.
 7. The thermal energy storage system as claimed in any one ofclaims 1 to 3 wherein the steel vessel has a peripheral wall thicknesswithin the range of about 6 mm to about 18 mm.
 8. The thermal energystorage system as claimed in any one of claims 1 to 3 wherein the steelvessel has a lower end wall of convex form and thickness within therange of about 6 mm to about 18 mm.
 9. The thermal energy storage systemas claimed in claim 8 wherein the lower end of the cavity is formed as aconcavity that complements the convex form of the lower end of thevessel.
 10. The thermal energy storage system as claimed in any one ofclaims 1 to 3 wherein the steel vessel has an upper end wall of convexform and a thickness of at least 20 mm.
 11. The thermal energy storagesystem as claimed in any one of claims 1 to 3 wherein the upper end ofthe vessel is anchored to surrounding rock strata by rock bolts.
 12. Thethermal energy storage system as claimed in claim 3 wherein the fillermaterial is selected as one having a coefficient of thermal expansionapproximately equal to that of the steel from which the vessel isformed.
 13. The thermal energy storage system as claimed in claim 3wherein the filler material is selected as one having a resiliencypermitting compression and expansion with thermally induced changes inthe dimensions of the vessel.
 14. The thermal energy storage system asclaimed in claim 3 wherein the filler material comprises a mineralmaterial that solidifies in situ.
 15. The thermal energy storage systemas claimed in claim 3 wherein the filler material comprises a mineralmaterial in particulate form.
 16. The thermal energy storage system asclaimed in any one of claims 1 to 3 and comprising a plurality of thevessels, with each vessel being located within its own cavity.
 17. Thethermal energy storage system as claimed in claim 3 wherein a pluralityof the vessels is located in a matrix of the filler material.
 18. Thethermal energy storage system as claimed in claim 16 wherein each vesselhas a diameter of approximately 1.5 m and the vessels are separated byapproximately 3.0 m.
 19. The thermal energy storage system as claimed inclaim 16 wherein the vessels are clustered in closely-spacedrelationship.
 20. The thermal energy storage system as claimed in claim17 wherein the vessels have a circular cross-section.
 21. The thermalenergy storage system as claimed in claim 17 wherein the vessels have apolygonal cross-section.
 22. The thermal energy storage system asclaimed in any one of claims 1 to 3 incorporated in a power generatingplant having a solar energy collector system and a steam turbineconnected in circuit with the thermal energy storage system.
 23. Thethermal energy storage system as claimed in claim 22 wherein the solarenergy collector system comprises a field of arrayed ground-mountedpivotal reflectors that in use are driven to track the sun and reflectincident solar radiation to at least one elevated receiver system.
 24. Amethod of storing thermal energy wherein water at a high temperature ismaintained under pressure within a cylindrical steel vessel which ispositioned within a cavity which is formed within ground that isconstituted by geologically stable consolidated rock, wherein the vesselis surrounded peripherally by a containment material and wherein thevessel has a peripheral wall that functions as a liner for thecontainment material and internal pressure-induced forces aretransferred from the vessel to the containment material by way of theperipheral wall.
 25. (canceled)
 26. (canceled)